Electrode and lithium-ion battery

ABSTRACT

The present application provides an electrode and a lithium-ion battery. The electrode comprises: a current collector; a first active material layer comprising a first active material; and a second active material layer comprising a second active material; wherein the first active material layer is arranged between the current collector and the second active material layer. The first active material layer is formed on at least one surface of the current collector, and a ratio of an average particle size of the second active material to an average particle size of the first active material is from 1:1 to 40:1. The active material layer is used in the present application to ensure that the lithium-ion battery does not generate a short circuit when pressed by an external force, thereby ensuring the mechanical safety performance of the lithium-ion battery.

TECHNICAL FIELD

The present application relates to battery, and more particularly to anelectrode and a lithium-ion battery.

BACKGROUND

Lithium-ion batteries have entered the daily life with the advancementof science and technology and the improvement of environmentalprotection requirements. With the rapid popularization of lithium-ionbatteries, safety problems caused by external forces puncturinglithium-ion batteries occasionally occur on the user's side, and theirsafety performance has been increasingly received attention by people,especially the continued fermentation of some cell phone explosions,causing users, sales backends, and lithium-ion battery manufacturers allplace new demands on the safety performance of lithium-ion batteries.

Current methods for improving the safety of lithium-ion batteries are atthe expense of the energy density of lithium-ion batteries. Therefore,there is an urgent need to provide a technology that can significantlyimprove the safety performance of lithium-ion batteries under conditionsof higher energy density.

SUMMARY OF THE APPLICATION

In the examples of the present application, a double layer design usedfor the active material layer in the electrode can avoid the internalshort circuit of the lithium-ion battery caused by the external forceand will not cause the battery to fail.

Some examples of the present application provide an electrodecomprising: a current collector; a first active material layer,comprising a first active material; and a second active material layer,comprising a second active material; wherein the first active materiallayer is arranged between the current collector and the second activematerial layer, the first active material layer is formed on at leastone surface of the current collector, ands ratio of an average particlesize of the second active material to an average particle size of thefirst active material is from 1:1 to 40:1.

In above electrode, the first active material has an average particlesize in a range of 0.2 μm to 15 μm. The average particle size (Dv50)refers to a particle size that reaches a volume accumulation of 50% fromthe small particle size side in a volume-based granularity distribution.

In above electrode, a particle size of 90% accumulative volume of thefirst active material is less than 40 μm. A particle size of 90%accumulative volume (Dv90) refers to a particle size that reaches avolume accumulation of 90% from the small particle size side in avolume-based granularity distribution.

In above electrode, the first active material layer has a thickness of 2μm to 30 μm.

In above electrode, the second active material layer has a thicknessequal to or more than 30 μm.

In above electrode, the first active material comprises one of lithiumcobaltate; lithium manganate, lithium nickelate, lithium nickel cobaltmanganese oxide, lithium iron phosphate, lithium iron manganesephosphate, lithium vanadium phosphate, lithium vanadyl phosphate,lithium-rich manganese-based materials, lithium nickel cobalt aluminateand lithium titanate, and combinations thereof.

In above electrode, the second active material comprises one of lithiumcobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobaltaluminum oxide, lithium-rich manganese-based material, lithium ironphosphate, and combinations thereof.

In above electrode, the first active material comprises one ofartificial graphite, natural graphite, mesocarbon microbeads, softcarbon, hard carbon, silicon, silicon carbide, lithium titanate, andcombinations thereof.

In above electrode, each of the first active material layer and thesecond active material layer further comprises a binder, and the bindercomprises one selected from polyvinylidene fluoride, vinylidenefluoride-hexafluoropropylene copolymer, polyimide, polyacrylonitrile,polyacrylates, polyacrylic acids, polyacrylic acid salt,carboxymethylcellulose sodium, polyethylene pyrrolidone, polyvinylether, polymethyl methacrylate, polytetrafluoroethylene,polyhexafluoropropylene and styrene butadiene rubber, and combinationsthereof.

Some examples of the present application further provide a lithium-ionbattery comprising above electrode s.

The double-layered active material layer is used for the active materiallayer in the electrode of the present application to ensure that thelithium-ion battery does not generate a short circuit when pressed by anexternal force, thereby ensuring the mechanical safety performance ofthe lithium-ion battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of the structure of a positive electrode.

FIG. 2 shows a schematic view of the structure of a positive electrodeaccording to some examples of the present application.

DETAILED DESCRIPTION OF THE EXAMPLE

The following examples may enable those skilled in the art to more fullyunderstand the present application, but do not limit the presentapplication in any way.

The double-layered active material layer is used for the active materiallayer in the electrode in the examples of the present application toensure that the lithium-ion battery does not generate a short circuitwhen pressed by an external force (the corresponding test method beingin a nail test), thereby ensuring the mechanical safety performance ofthe lithium-ion battery (50% pass the nail test). The technical means ofthe present application can be applied to a lithium-ion battery and playa great safety role in a lithium-ion battery application terminal. Itshould be noted that the present application takes the positiveelectrode as an example, and the negative electrode can also realize thetechnical solution.

In the process of nail penetration for lithium-ion battery, four shortcircuits usually occur: positive electrode active materiallayer-negative electrode active material layer, positive electrodeactive material layer-negative electrode current collector (usuallycopper foil), positive electrode current collector (usually aluminumfoil)-negative electrode current collector, positive electrode currentcollector-negative electrode active material layer. In these fourshort-circuit modes, the short circuit mode of the positive electrodecurrent collector-negative electrode active material layer is the mostdangerous among the four short circuit modes because the short circuitpower at the time of a short circuit is very large. Therefore,preferentially avoiding such a short circuit mode of the positiveelectrode current collector-negative electrode active material layer isthe most direct means for improving the safety of the lithium-ionbattery penetrating through nails.

In the present application, the short circuit of the positive currentcollector-negative electrode active material layer is avoided. That isto say, a coating with a higher resistivity is provided on the surfaceof the negative electrode active material layer, or a coating with ahigher resistivity is provided on the surface of the positive electrodecurrent collector to avoid direct contact between the negative electrodeactive material layer and the positive electrode current collector so asto prevent the most dangerous short circuit mode from occurring. Thecoating provided on the surface of the negative electrode activematerial layer or the surface of the separator is usually a ceramiclayer. By increasing the thickness of the ceramic layer, a short circuitbetween the negative electrode active material layer and the positiveelectrode current collector can be avoided. In order to avoid thisshort-circuit pattern between the negative electrode active materiallayer and the positive electrode current collector, the thickness ofthese coatings should not be too thin (generally at least greater thanthe surface roughness of the negative electrode active material layersurface), which will bring about the loss of energy density of thelithium-ion battery. In addition, these coatings increase thetransmission distance of lithium ions from the positive electrode to thenegative electrode, increase the transmission impedance of thelithium-ion battery, and greatly deteriorate the dynamic performance ofthe lithium-ion battery. And due to the large damage to the lithium-ionbattery during the nail process, part of the coating will fall offduring the nail process, thus affecting the stability of the nailprotection effect.

One technical means is to allow the surface of the positive electrodecurrent collector to adhere to a material with a higher resistivity toavoid direct contact between the positive electrode current collectorand the negative electrode active material layer during the nailprocess. It is a common practice to reduce the particle size of thepositive electrode active material and increase the binder content ofthe positive electrode active material layer. The decrease of theparticle size of the positive electrode active material will reduce thecycle performance of the lithium-ion battery, and increasing the bindercontent in the positive electrode active material layer will make thepositive electrode become brittle, resulting in a greater effects on theproduction process and the energy density of the lithium-ion battery.

As shown in FIG. 1, a schematic view of a positive electrode is shown.The positive electrode current collector 1 is located between two activematerial layers 2. FIG. 2 shows a double-layered active material layerstructure of the present application, i.e., an additional activematerial layer 3 is formed between the active material layer 2 and thepositive electrode collector 1. The material and formulation of theactive material layer 3 are optimized so that the adhesive force betweenthe active material layer 3 and the positive electrode current collector1 is increased, thereby protecting the positive electrode currentcollector 1, and the active material layer 3 will not come off duringthe nail process. Also, the short-circuit mode of the positive electrodecurrent collector-negative electrode active material layer does notoccur during the nail process, thereby ensuring the safety performanceof the lithium-ion battery. For better differentiation, the activematerial layer 3 is hereinafter referred to as a first active materiallayer, and the active material layer 2 is referred to as a second activematerial layer. This is merely for better description and does not limitthe present application.

The second active material layer 2 and the first active material layer 3of the present application each contain a positive active material whencompared with coating a non-conductive or poorly-conductive materiallayer between the positive electrode current collector and the activematerial layer, so that both the first and second active material layerscan provide energy, and their energy density is high. The design of thepresent application has no effect on the conductive properties of theactive material layer, while using a non-conductive or poorly conductivecoating for lithium-ion batteries may exert greater influence on theelectronic conductance capability of lithium-ion batteries, so that thenormal charge and discharge of the lithium-ion battery will be affected.

The positive electrode active material of the first active materiallayer in the examples of the present application comprises one oflithium cobaltate, lithium manganate, lithium nickelate, lithium nickelcobalt manganese oxide, lithium iron phosphate, lithium iron manganesephosphate, lithium vanadium phosphate, lithium vanadyl phosphate,lithium-rich manganese-based materials, lithium nickel cobalt aluminateand lithium titanate, and combinations thereof. The positive electrodeactive material of the second active material layer comprises thecombination of one or more selected from the group consisting of lithiumcobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobaltaluminum oxide, lithium-rich manganese-based material, lithium ironphosphate. The positive electrode active material layer is a two-layerstructure, and there are two positive electrode active material layerson the surface of the positive electrode current collector. Although thepositive electrode active material of the two active material layersshown in FIG. 2 has a clear interface, the interfaces of the two activematerial layers are bonded to each other without obvious interface bythe embedding of the upper and lower layer particles and the binderbetween the two active material layers.

The active material layer of the double-layer structure protects thepositive electrode current collector and the first active material layerplays a leading role. In order to achieve the protection of the positiveelectrode current collector, the positive electrode active material ofthe first active material layer is required to have a small particlesize so as to achieve higher coverage and adhesion on the positiveelectrode current collector (aluminum foil). In order to ensure thecovering and bonding of the positive electrode current collector whilereducing the influence of the first active material layer on the wholeactive material layer (the first active material layer has a lowercompaction density than the second active material layer, so it isnecessary to reduce the volume proportion of the first active materiallayer in the whole active material layer as much as possible). In thegranularity distribution of the positive electrode active material ofthe first active material layer on a volume basis, a particle size thatreaches volume accumulation of 50% from a small particle size (Dv50)ranges from 0.2 μm to 15 μm. And in the granularity distribution of thepositive electrode active material of the first active material layer ona volume basis, a particle size that reaches volume accumulation of 90%from a small particle size (Dv90) ranges below 40 μm. In order to ensurethe protective effect on the positive electrode current collector, thefirst active material layer needs to completely cover the positiveelectrode current collector. The smaller the particles of the positiveelectrode active material, the thinner the coating can be made while thecompaction density of the positive electrode active material of smallparticles will be lower, leading a certain impact on the energy densityof the lithium-ion battery.

In addition, considering that the first active material layer and thesecond active material layer also have some interactions, comprising theeffects of adhesion and electron conduction, and considering that thefirst active material layer cannot be crushed due to the force transferof the second active material layer particles during the cold pressing,so the particle size ratio of the second active material layer and thefirst active material layer is another important factor that affects theperformance of the lithium-ion battery.

In order to prevent the first active material layer from being damagedby the positive active material in the second active material layerduring the cold pressing and to ensure the maximum adhesion between thefirst active material layer and the second active material layer, theratio of Dv50 between the positive electrode active materials of thefirst active material layer and the second active material layer is in acertain range, i.e., Dv50 (of the second active material layer): Dv50(of the first active material layer)=1:1 to 40:1. The particles of thepositive active material of the second active material layer are toolarge, and thus the destructive effect on the first active materiallayer after the cold press increases, thereby resulting in the weakeningof the protective effect of the first active material layer on thepositive electrode current collector. In addition to the binding effectof the binder between the first active material layer and the secondactive material layer, the mutual riveting action of the positiveelectrode active material particles can also provide a certain bindingeffect. This bonding effect requires that the particle size differencebetween the particles is not too large. When the particles of the secondactive material layer are too large, this riveting effect is weakened,so that the interface between the layers becomes distinct or evenseparate.

In addition, the first active material layer needs a certain thicknessin order to achieve full coverage of the positive electrode currentcollector. In order to achieve this goal, the particle size of theactive material of the first active material layer has an upper limit.Since the particle size of the positive electrode active material in thefirst active material layer is small and the binder contained in thefirst active material layer is larger than the binder in the secondactive material layer, the thickness of the first active material layercannot be too large, otherwise it will reduce the energy density oflithium-ion batteries. Also, since the first active material layer needsto cover the positive electrode current collector, the thickness of thefirst active material layer needs to be controlled to 3 μm-40 μm, andthe thickness after cold pressing is 2 μm-30 μm. In particular, thethickness of the first active material layer is not less than Dv90 ofthe positive electrode active material in the first active materiallayer, which is to ensure the coverage of the first active materiallayer, thereby achieving complete protection of the positive electrodecurrent collector. Since the positive electrode active material of thefirst active material layer has a small particle size, the compactiondensity thereof will be relatively low. Therefore, in order to achieve ahigh energy density of the lithium-ion battery, the thickness of thesecond active material layer needs to be increased, and the thickness ofthe second active material layer is increased, e.g., to 30 μm or more.The thicker the second active material layer, the higher the energydensity of the lithium-ion battery.

The positive electrode is cold-pressed under a certain pressure, and theactive material layer is pressed into an electrode having a certainthickness. Since the active material layer of the present applicationhas a double-layer structure, the compaction density of the positiveelectrode can be divided into the compaction density of the secondactive material layer, the compaction density of the first activematerial layer, and the overall compaction density. To ensure the energydensity of a lithium-ion battery, it is required that the positiveelectrode has a higher compaction density, the compaction density of thefirst active material layer is greater than 2.8 g/cc, the compactiondensity of the second active material layer is greater than 3.3 g/cc andthe compaction density of the all active material layers is greater than3.2 g/cc.

In addition, in order to achieve higher bonding for the first activematerial layer, it is required that the first active material layercontains a certain amount of binder, and the binder comprises, but isnot limited to, one polyvinylidene fluoride, vinylidenefluoride-hexafluoropropylene copolymer, polyimide, polyacrylonitrile,polyacrylates, polyacrylic acids, polyacrylates, carboxymethylcellulosesodium, polyethylene pyrrolidone, polyvinyl ether, polymethylmethacrylate, polytetrafluoroethylene, polyhexafluoropropylene andstyrene butadiene rubber, and m combinations thereof. On the one hand,the binder allows the active material layer and the positive electrodecurrent collector to have a better adhesion, on the other hand, when thebinder content increases, the first active material layer will have alower compaction density. The mass content of the binder in the firstactive material layer is selected in the range of 1.5% to 6%, and themass content of the binder in the second active material layer isselected in the range of 0.5% to 4%.

Since the positive electrode active material in the positive electrodegenerally has a relatively common conductivity, the active materiallayer also contains a certain amount of a conductive agent, such ascarbon black (SP), carbon nanotubes (CNT), graphene, and the like. Theconductive agent increases its electrical conductivity, and the masscontent of the conductive agent is selected in the range from 0.5% to5%.

In addition, some other treatments may be performed on the first activematerial layer or the second active material layer, or some treatmentsmay be performed on the positive electrode current collector, such as aroughness treatment, a heat treatment, etc. The working principle oreffect comprises enhancing the adhesion of the current collector.Although such principle or effect is not described in detail in thisapplication, it is comprised in the scope of the present application.

The examples of the present application further provide a lithium-ionbattery comprising above positive electrode. The lithium-ion batterycomprises a positive electrode, a negative electrode, an separator, anelectrolyte, and the like. The negative electrode comprises a negativeelectrode current collector and a negative electrode active materiallayer coated on the negative electrode current collector, and thenegative electrode active material layer comprises a negative electrodeactive material, a conductive agent and a binder. The negative electrodecurrent collector may be a Cu foil, however, other negative electrodecurrent collectors commonly used in the art may be used. The conductiveagent and the binder of the negative electrode active material layer aresimilar to the conductive agent and the binder of the positive electrodeactive material layer described above, and will not be described herein.The negative electrode active material comprises, but is not limited to,one of soft carbon, hard carbon, mesocarbon microbeads (MCMB), silicon,silicon-carbon composites, lithium titanate, alloys, artificialgraphite, and natural graphite, and combinations thereof. Above negativeelectrode active material comprises a negative electrode active materialthat has been doped and/or coated in the prior art.

The separator comprises a polyethylene (PE) separator, a polypropylene(PP) separator, and the like. In addition, the separator comprises oneof a bare separation film, an inorganic particle-coated separation film,a polymer-coated separator, or a combination thereof, depending onwhether or not the surface of the separator comprises a coating and onthe type of the coating. The electrolyte comprises at least two ofdimethyl carbonate (DMC); ethyl methyl carbonate (EMC), diethylcarbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC),propyl propionate (PP). Further, the electrolyte may additionallycomprise at least one of vinylene carbonate (VC), fluoroethylenecarbonate (FEC), and dinitrile compounds as an electrolyte additive,wherein the dinitrile compound comprises succinonitrile (SN).

The positive electrode, the separator, and the negative electrode aresequentially wound or stacked into a bare cell, and then filled into,for example, an aluminum plastic film, and then the electrolyte isinjected, followed by chemical conversion and package. Then, theprepared lithium-ion battery is subjected to performance test and cycletest.

It will be understood by those skilled in the art that above-describedmethod for producing a lithium-ion battery is only an example. Other mmethods commonly used in the art can be used without departing from thedisclosure of the present application.

The electrode of the present application can be used in lithium-ionbatteries of different structures. In the examples, a wound lithium-ionbattery is taken as an example, but the positive electrode of thepresent application can be applied to lithium ions of laminatedstructures, multi-electrode tab structures, and the like, all of whichare comprised in the scope of the present application.

The electrode of the present application may be other types ofelectrode. In the examples, a positive electrode is taken as an example,but the electrode of the present application may be a negativeelectrode, all of which are comprised in the scope of the presentapplication.

The electrode of the present application can be used in lithium-ionbatteries of different types. In the examples, a soft packagelithium-ion battery is taken as an example, but the electrode of thepresent application can be applied to other lithium-ion batteries suchas square aluminum shell batteries, cylindrical batteries, and the like,all of which are comprised in the scope of the present application.

Some specific examples and comparative examples are listed below tobetter illustrate this application.

COMPARATIVE EXAMPLE 1

An aluminum foil is used as the positive electrode collector, and alayer of lithium cobaltate slurry composed of 97.8 wt % LiCoO₂(LCO), 0.8wt % polyvinylidene fluoride (PVDF) and 1.4 wt % conductive carbon blackis evenly coated on the surface of aluminum foil. Then drying wasperformed at 85° C., followed by cold pressing, cutting, slitting, anddrying under a vacuum condition of 85° C. for 4 h to prepare a positiveelectrode, wherein the thickness of the coating is 63 μm.

A copper foil is used as a negative electrode collector, and a layer ofgraphite slurry composed of 97.7 wt % artificial graphite, 1.3 wt %carboxymethyl cellulose sodium (CMC) and 1.0 wt % styrene butadienerubber (SBR) is evenly coated on the surface of copper foil. Then dryingwas performed at 85° C., followed by cold pressing, cutting, slitting,and drying under a vacuum condition of 85° C. for 4 h to prepare anegative electrode.

The positive electrode and the negative electrode are wound after beingslit, and the positive electrode and the negative electrode areseparated by a polyethylene separator to prepare a wound bare cell.After the bare cell is top-side sealed, spray-coded, vacuum-dried,filled with electrolyte, and allowed to stand at high temperatures forchemical conversion and capacity, a finished lithium-ion battery can beobtained.

EXAMPLE 1

An aluminum foil is used as the positive electrode collector, and alayer of small particle lithium nickel cobalt manganese oxide slurrycomposed of 95.8 wt % lithium nickel cobalt manganese oxide, 2.8 wt %polyvinylidene fluoride (PVDF) and 1.4 wt % conductive carbon black isevenly coated on the surface of aluminum foil for drying under 85° C.; alayer of lithium cobalt oxide slurry composed of 97.8 wt % LiCoO₂(LCO),0.8 wt % polyvinylidene fluoride (PVDF) and 1.4 wt % conductive carbonblack is evenly coated as a second active material layer on the firstactive material layer coated with the lithium nickel cobalt manganeseoxide slurry; then drying was performed at 85° C., followed by coldpressing, cutting, slitting, and drying under a vacuum condition of 85 Vfor 4 h to prepare a positive electrode. Wherein, Dv50 of the firstactive material (lithium nickel cobalt manganese oxide) of the firstactive material layer is 0.2 μm, and Dv90 of the first active material(lithium nickel cobalt manganese oxide) of the first active materiallayer is 20 μm. The thickness of the first active material layer is 25μm, the ratio of Dv50 between the second active material to the firstactive material is 2:1, and the thickness of the second active materiallayer is 54 μm.

A copper foil is used as a negative electrode collector, and a layer ofgraphite slurry composed of 97.7 wt % artificial graphite, 1.3 wt %carboxymethyl cellulose sodium (CMC) and 1.0 wt % styrene butadienerubber (SBR) is evenly coated on the surface of copper foil. Then dryingwas performed at 85° C., followed by cold pressing, cutting, slitting,and drying under a vacuum condition of 85° C. for 4 h to prepare anegative electrode.

The positive electrode and the negative electrode are wound after beingslit, and the positive electrode and the negative electrode areseparated by a polyethylene separator to prepare a wound bare cell.After the bare cell is top-side sealed, spray-coded, vacuum-dried,filled with electrolyte, and allowed to stand at high temperatures forchemical conversion and capacity, a finished lithium-ion battery can beobtained.

EXAMPLE 2

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 2 is 0.5 μm.

EXAMPLE 3

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 3 is 1 μm.

EXAMPLE 4

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 4 is 3 μm.

EXAMPLE 5

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 5 is 5 μm.

EXAMPLE 6

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 6 is 7 μm.

EXAMPLE 7

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 7 is 9 μm.

EXAMPLE 8

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 8 is 11 μm.

EXAMPLE 9

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 9 is 15 μm.

EXAMPLE 10

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 10 is 16 μm.

EXAMPLE 11

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 11 is 7 μm and Dv90 of thefirst active material layer is 8 μm.

EXAMPLE 12

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 12 is 7 μm and Dv90 of thefirst active material layer is 10 μm.

EXAMPLE 13

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 13 is 7 μm and Dv90 of thefirst active material layer is 15 μm.

EXAMPLE 14

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 14 is 7 μm and Dv90 of thefirst active material layer is 25 μm.

EXAMPLE 15

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 15 is 7 μm and Dv90 of thefirst active material layer is 40 μm.

EXAMPLE 16

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 16 is 1 μm and Dv90 of thefirst active material layer is 2 μm.

EXAMPLE 17

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 17 is 1 μm and Dv90 of thefirst active material layer is 5 μm.

EXAMPLE 18

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 18 is 1 μm and Dv90 of thefirst active material layer is 10 μm.

EXAMPLE 19

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 19 is 1 μm and Dv90 of thefirst active material layer is 15 μm.

EXAMPLE 20

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 20 is 1 μm and Dv90 of thefirst active material layer is 20 μm.

EXAMPLE 21

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 21 is 7 μm and Dv90 of thefirst active material layer is 45 μm.

EXAMPLE 22

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 22 is 7 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 1:1.

EXAMPLE 23

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 23 is 7 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 5:1.

EXAMPLE 24

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 24 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 1:1.

EXAMPLE 25

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 25 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 2:1.

EXAMPLE 26

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 26 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 5:1.

EXAMPLE 27

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 27 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 10:1.

EXAMPLE 28

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 28 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 20:1.

EXAMPLE 29

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 29 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 40:1.

EXAMPLE 30

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 30 is 1 μm and the ratioof Dv50 between the second active material layer to the first activematerial layer is 45:1.

EXAMPLE 31

It is the same as the preparation method of Example 1 except that Dv50of the first active material layer in Example 31 is 1 μm and thethickness of the first active material layer is 2 μm.

EXAMPLE 32

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 32 is 1 μm and thethickness of the first active material layer is 5 μm.

EXAMPLE 33

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 33 is 1 μm and thethickness of the first active material layer is 7 μm.

EXAMPLE 34

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 34 is 1 μm and thethickness of the first active material layer is 10 μm.

EXAMPLE 35

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 35 is 1 μm and thethickness of the first active material layer is 15 μm.

EXAMPLE 36

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 36 is 1 μm and thethickness of the first active material layer is 20 μm.

EXAMPLE 37

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 37 is 1 μm and thethickness of the first active material layer is 30 μm.

EXAMPLE 38

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 38 is 7 μm and thethickness of the second active material layer is 30 μm.

EXAMPLE 39

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 39 is 7 μm and thethickness of the second active material layer is 40 μm.

EXAMPLE 40

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 40 is 7 μm and thethickness of the second active material layer is 50 μm.

EXAMPLE 41

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 41 is 7 μm and thethickness of the second active material layer is 60 μm.

EXAMPLE 42

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 42 is 7 μm and thethickness of the second active material layer is 70 μm.

EXAMPLE 43

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 43 is 7 μm and thethickness of the second active material layer is 80 μm.

EXAMPLE 44

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 44 is 7 μm and thethickness of the second active material layer is 100 μm.

EXAMPLE 45

It is the same as the preparation method of Example 1, except that Dv50of the first active material layer in Example 45 is 7 μm.

EXAMPLE 46

It is the same as the preparation method of Example 1, except that thepositive electrode active material of the first active material layer inExample 46 is lithium cobaltate and Dv50 of the first active materiallayer is 7 μm.

EXAMPLE 47

It is the same as the preparation method of Example 1, except that thepositive electrode active material of the first active material layer inExample 47 is lithium manganese oxide and Dv50 of the first activematerial layer is 7 μm.

EXAMPLE 48

It is the same as the preparation method of Example 1, except that thepositive electrode active material of the first active material layer inExample 48 is lithium iron phosphate and Dv50 of the first activematerial layer is 7 μm.

EXAMPLE 49

It is the same as the preparation method of Example 1, except that thepositive electrode active material of the first active material layer inExample 49 is lithium nickel cobalt aluminate and Dv50 of the firstactive material layer is 7 μm.

EXAMPLE 50

It is the same as the preparation method of Example 1, except that thepositive electrode active material of the first active material layer inExample 50 is lithium titanate and Dv50 of the first active materiallayer is 7 μm.

Next, the passing rate of nail penetration and energy density oflithium-ion batteries are tested.

1. Method for Nail Test:

Place the lithium-ion battery in a 25° C. thermostat and allow it tostand for 30 minutes to allow the lithium-ion battery to reach aconstant temperature. The thermostated lithium-ion battery is charged ata constant current of 0.5 C to a voltage of 4.4 V, and then charged at aconstant voltage of 4.4 V to a current of 0.025 C. The fully-chargedlithium-ion battery is transferred to a nailing tester and the testambient temperature is maintained at 25° C.±2±2, and then a steel nailwith a 4 mm diameter is used to penetrate through the center of thelithium-ion battery at a uniform speed of 30 mm/s and keep the steelnail for 300 s. If the lithium-ion battery does not fire and explode, itis marked as PASS. Ten lithium-ion batteries are tested each time, andthe number of lithium-ion batteries passed the nail test is used as anindicator to evaluate the safety performance of the lithium-ion battery.

2. Method for Testing Energy Density:

Place the lithium-ion battery in a 25° C. thermostat and allow it tostand for 30 minutes to allow the lithium-ion battery to reach aconstant temperature. The thermostated lithium-ion battery is charged ata constant current of 0.5 C to a voltage of 4.4 V, and then charged at aconstant voltage of 4.4 V to a current of 0.05 C, discharged at 0.5 C toa voltage of 3.0 V, and the discharge energy is recorded.

Energy density=discharge energy/(length*width*thickness of lithium-ionbattery).

Experimental parameters and measurement results of the respectiveexamples and comparative examples are shown in Table 1 below.

TABLE 1 Mass Mass Dv50 content content ratio of of (the ThicknessThickness binder binder Dv50/ Dv90/ second of of of of μm μm active thethe the the (the (the material:the first second first second nail firstfirst first active active active active penetration Energy The firstactive active active material material material material PASS densityactive material material) material) material) (/μm) μm (wt %) (wt %)rate (Wh/L) Examples 1 lithium nickel cobalt 0.2 20 2:1 25 54 2.8 0.810/10 721 manganese oxide 2 lithium nickel cobalt 0.5 20 2:1 25 54 2.80.8 10/10 721 manganese oxide 3 lithium nickel cobalt 1 20 2:1 25 54 2.80.8 10/10 721 manganese oxide 4 lithium nickel cobalt 3 20 2:1 25 54 2.80.8 10/10 729 manganese oxide 5 lithium nickel cobalt 5 20 2:1 25 54 2.80.8 10/10 735 manganese oxide 6 lithium nickel cobalt 7 20 2:1 25 54 2.80.8 10/10 738 manganese oxide 7 lithium nickel cobalt 9 20 2:1 25 54 2.80.8 10/10 741 manganese oxide 8 lithium nickel cobalt 11 20 2:1 25 542.8 0.8  8/10 745 manganese oxide 9 lithium nickel cobalt 15 20 2:1 2554 2.8 0.8  6/10 750 manganese oxide 10 lithium nickel cobalt 16 20 2:125 54 2.8 0.8  5/10 752 manganese oxide 11 lithium nickel cobalt 7 8 2:125 54 2.8 0.8 10/10 738 manganese oxide 12 lithium nickel cobalt 7 102:1 25 54 2.8 0.8 10/10 738 manganese oxide 13 lithium nickel cobalt 715 2:1 25 54 2.8 0.8 10/10 738 manganese oxide 6 lithium nickel cobalt 720 2:1 25 54 2.8 0.8 10/10 738 manganese oxide 14 lithium nickel cobalt7 25 2:1 25 54 2.8 0.8  9/10 738 manganese oxide 15 lithium nickelcobalt 7 40 2:1 25 54 2.8 0.8  9/10 738 manganese oxide 16 lithiumnickel cobalt 1 2 2:1 25 54 2.8 0.8 10/10 721 manganese oxide 17 lithiumnickel cobalt 1 5 2:1 25 54 2.8 0.8 10/10 721 manganese oxide 18 lithiumnickel cobalt 1 10 2:1 25 54 2.8 0.8 10/10 721 manganese oxide 19lithium nickel cobalt 1 15 2:1 25 54 2.8 0.8 10/10 721 manganese oxide20 lithium nickel cobalt 1 20 2:1 25 54 2.8 0.8 10/10 721 manganeseoxide 21 lithium nickel cobalt 7 45 2:1 25 54 2.8 0.8  3/10 737manganese oxide 22 lithium nickel cobalt 7 20 1:1 25 54 2.8 0.8 10/10741 manganese oxide 6 lithium nickel cobalt 7 20 2:1 25 54 2.8 0.8 10/10738 manganese oxide 23 lithium nickel cobalt 7 20 5:1 25 54 2.8 0.8 9/10 725 manganese oxide 24 lithium nickel cobalt 1 20 1:1 25 54 2.80.8 10/10 721 manganese oxide 25 lithium nickel cobalt 1 20 2:1 25 542.8 0.8 10/10 721 manganese oxide 26 lithium nickel cobalt 1 20 5:1 2554 2.8 0.8 10/10 721 manganese oxide 27 lithium nickel cobalt 1 20 10:1 25 54 2.8 0.8 10/10 717 manganese oxide 28 lithium nickel cobalt 1 2020:1  25 54 2.8 0.8  9/10 713 manganese oxide 29 lithium nickel cobalt 120 40:1  25 54 2.8 0.8  7/10 709 manganese oxide 30 lithium nickelcobalt 1 20 45:1  25 54 2.8 0.8  2/10 697 manganese oxide 31 lithiumnickel cobalt 1 20 2:1 2 54 2.8 0.8 10/10 721 manganese oxide 32 lithiumnickel cobalt 1 20 2:1 5 54 2.8 0.8 10/10 719 manganese oxide 33 lithiumnickel cobalt 1 20 2:1 7 54 2.8 0.8 10/10 718 manganese oxide 34 lithiumnickel cobalt 1 20 2:1 10 54 2.8 0.8 10/10 715 manganese oxide 35lithium nickel cobalt 1 20 2:1 15 54 2.8 0.8 10/10 711 manganese oxide36 lithium nickel cobalt 1 20 2:1 20 54 2.8 0.8 10/10 708 manganeseoxide 37 lithium nickel cobalt 1 20 2:1 30 54 2.8 0.8 10/10 698manganese oxide 38 lithium nickel cobalt 7 20 2:1 25 30 2.8 0.8 10/10723 manganese oxide 39 lithium nickel cobalt 7 20 2:1 25 40 2.8 0.810/10 734 manganese oxide 40 lithium nickel cobalt 7 20 2:1 25 50 2.80.8 10/10 735 manganese oxide 6 lithium nickel cobalt 7 20 2:1 25 54 2.80.8 10/10 738 manganese oxide 41 lithium nickel cobalt 7 20 2:1 25 602.8 0.8 10/10 743 manganese oxide 42 lithium nickel cobalt 7 20 2:1 2570 2.8 0.8 10/10 751 manganese oxide 43 lithium nickel cobalt 7 20 2:125 80 2.8 0.8 10/10 766 manganese oxide 44 lithium nickel cobalt 7 202:1 25 100 2.8 0.8 10/10 778 manganese oxide 45 lithium nickel cobalt 720 2:1 25 54 2.8 0.8 10/10 738 manganese oxide 46 lithium cobaltate 7 202:1 25 54 2.8 0.8 10/10 743 47 lithium manganese 7 20 2:1 25 54 2.8 0.810/10 728 oxide 48 lithium iron 7 20 2:1 25 54 2.8 0.8 10/10 730phosphate 49 lithium nickel cobalt 7 20 2:1 25 54 2.8 0.8 10/10 735aluminate 50 lithium titanate 7 20 2:1 25 54 2.8 0.8 10/10 723 Com-parative Example 1 / / / / / 79 / 0.8  0/10 747

Comparing Comparative Example 1 with Examples 1-50, it can be seen thatthrough the use of the active material layer having a double layerstructure, the nail penetration PASS rate of the lithium-ion battery hasbeen improved with a different extent, and the energy density is notsubstantially affected.

From Examples 1-9, it can be seen that as the Dv50 of the first activematerial layer increases, the energy density also increases, but thenthe nail penetration PASS rate is reduced. By comparing Examples 1-10,it can be seen that when the Dv50 of the first active material layer isgreater than 15 μm, the nail penetration PASS rate of the lithium-ionbattery is reduced to a low level.

From Examples 11-20 and 6, it can be seen that as the Dv90 of the firstactive material layer increases, the energy density is almost constant,but then the nail penetration PASS rate of the lithium-ion battery isreduced. By comparing Example 6 with Examples 11-21, it can be seen thatwhen the Dv90 of the first active material layer of the lithium-ionbattery is greater than 40 μm, the nail penetration PASS rate of thelithium-ion battery is reduced to a low level.

By comparing Example 6 with Examples 22-30, it can be seen that as theratio of the positive electrode active material Dv50 of the secondactive material layer and the first active material layer increases,both the energy density and the nail penetration PASS rate decrease, andthe second active material decreases. When the ratio of the positiveelectrode active material Dv50 between the first and second activematerial layer exceeds 40, the nail penetration PASS rate of thelithium-ion battery is reduced to a low level.

According to Examples 31-37, it can be seen that when the thickness ofthe first active material layer is in a range of 2 μm to 30 μm, the nailpenetration performance of the lithium-ion battery is maintained well.

By comparing Example 6 with Examples 38-44, it can be seen that as thethickness of the second active material layer increases, the nailpenetration performance of the lithium-ion battery remains substantiallyunchanged, but the energy density increases.

From Examples 45-50, it can be seen that the positive active material ofthe first active material layer is selected from different materials,all of the lithium-ion batteries have excellent nail penetrationperformance, and the energy density varies slightly with differentmaterial.

Those skilled in the art should understand that the above examples aremerely exemplary examples, and various changes, substitutions, andchanges can be made without departing from the spirit and scope of thepresent application.

What is claimed is:
 1. An electrode, comprising: a current collector; afirst active material layer, comprising a first active material; and asecond active material layer, comprising a second active material;wherein the first active material layer is arranged between the currentcollector and the second active material layer, the first activematerial layer is formed on at least one surface of the currentcollector, and a ratio of an average particle size of the second activematerial to an average particle size of the first active material isfrom 1:1 to 40:1.
 2. The electrode according to claim 1, wherein thefirst active material has an average particle size in a range of 0.2 μmto 15 μm.
 3. The electrode according to claim 1, wherein a particle sizeof 90% accumulative volume of the first active material is less than 40μm.
 4. The electrode according to claim 1, wherein the first activematerial layer has a thickness of 2 μm to 30 μm.
 5. The electrodeaccording to claim 1, wherein the second active material layer has athickness equal to or more than 30 μm.
 6. The electrode according toclaim 1, wherein the first active material comprises one of lithiumcobaltate, lithium manganate, lithium nickelate, lithium nickel cobaltmanganese oxide, lithium iron phosphate, lithium iron manganesephosphate, lithium vanadium phosphate, lithium vanadyl phosphate,lithium-rich manganese-based materials, lithium nickel cobalt aluminateand lithium titanate, and combinations thereof.
 7. The electrodeaccording to claim 1, wherein the second active material comprises oneof lithium cobaltate, lithium nickel cobalt manganese oxide, lithiumnickel cobalt aluminum oxide, lithium-rich manganese-based material,lithium iron phosphate, and combinations thereof.
 8. The electrodeaccording to claim 1, wherein the first active material comprises one ofartificial graphite, natural graphite, mesocarbon microbeads, softcarbon, hard carbon, silicon, silicon carbide, lithium titanate, andcombinations thereof.
 9. The electrode according to claim 1, whereineach of the first active material layer and the second active materiallayer further comprises a binder, and the binder comprises one ofpolyvinylidene fluoride, vinylidene fluoride-hexafluoropropylenecopolymer, polyamide, polyacrylonitrile, polyacrylates, polyacrylicacids, polyacrylic acid salt, carboxymethylcellulose sodium,polyethylene pyrrolidone, polyvinyl ether, polymethyl methacrylate,polytetrafluoroethylene, polyhexafluoropropylene and styrene butadienerubber, and combinations thereof.
 10. A lithium-ion battery comprisingan electrode, wherein the electrode comprises: a current collector; afirst active material layer, comprising a first active material; and asecond active material layer, comprising a second active material;wherein the first active material layer is arranged between the currentcollector and the second active material layer, the first activematerial layer is formed on at least one surface of the currentcollector, and a ratio of an average particle size of the second activematerial to an average particle size of the first active material isfrom 1:1 to 40:1.
 11. The lithium-ion battery according to claim 10,wherein the first active material has an average particle size in arange of 0.2 μm to 15 μm.
 12. The lithium-ion battery according to claim10, wherein a particle size of 90% accumulative volume of the firstactive material is less than 40 μm.
 13. The lithium-ion batteryaccording to claim 10, wherein the first active material layer has athickness of 2 μm to 30 μm.
 14. The lithium-ion battery according toclaim 10, wherein the second active material layer has a thickness equalto more than 30 μm.
 15. The lithium-ion battery according to claim 10,wherein the first active material comprises one of lithium cobaltate,lithium manganate, lithium nickelate, lithium nickel cobalt manganeseoxide, lithium iron phosphate, lithium iron manganese phosphate, lithiumvanadium phosphate, lithium vanadyl phosphate, lithium-richmanganese-based materials, lithium nickel cobalt aluminate and lithiumtitanate, and combinations thereof.
 16. The lithium-ion batteryaccording to claim 10, wherein the second active material comprises oneof lithium cobaltate, lithium nickel cobalt manganese oxide, lithiumnickel cobalt aluminum oxide, lithium-rich manganese-based material,lithium iron phosphate, and combinations thereof.
 17. The lithium-ionbattery according to claim 10, wherein the first active materialcomprises one of artificial graphite, natural graphite, mesocarbonmicrobeads, soft carbon, hard carbon, silicon, silicon carbide, lithiumtitanate, and combinations thereof.
 18. The lithium-ion batteryaccording to claim 10, wherein each of the first active material layerand the second active material layer further comprises a binder, and thebinder comprises one polyvinylidene fluoride, vinylidenefluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile,polyacrylates, polyacrylic acids, polyacrylic acid salt,carboxymethylcellulose sodium, polyethylene pyrrolidone, polyvinylether, polymethyl methacrylate, polytetrafluoroethylene,polyhexafluoropropylene and styrene butadiene rubber, and combinationsthereof.